The present invention generally relates to electronic apparatuses, and more particularly to manufacturing of such an apparatus having a substrate on which a wiring pattern is formed and one or more electric or electronic components are mounted.
Electronic apparatuses including those for microwave applications generally include various components such as capacitors, resistors, transistors and integrated circuits mounted upon a common print circuit board.
When producing such electronic apparatuses in large numbers, a number of identical print circuit boards are formed such that the print circuit boards carry identical circuit patterns thereon, and components such as capacitors, resistors, transistors or integrated circuits are mounted thereon in contact with the corresponding circuit patterns. Further, by passing the print circuit board through a reflowing furnace together with the components thereon, the solder paste that has been screen-printed on the circuit patterns causes a reflowing, and the components are soldered firmly upon the corresponding circuit patterns. Further, interconnection leads are provided on the print circuit board in connection with various input and output electrode pads as well as in connection with various power electrode pads and ground pads. In the recent semiconductor circuits where the components are mounted with high mounting density, the print circuit board may further be provided with a heat sink for dissipating heat produced by the components.
In order to produce such electronic apparatuses in large numbers and with high throughput of production, it is proposed to mount the interconnection leads on the print circuit boards in a state such that the interconnection leads are formed on a common lead frame as an integral part thereof. The lead frame may include a heat sink also as a part thereof.
FIGS. 1A and 1B show such a conventional process for mounting interconnection leads 13 upon print circuit boards 11a-11n, wherein FIG. 1A shows the top side of the print circuit boards 11a-11n while FIG. 1B shows bottom side thereof.
Referring to FIGS. 1A and 1B, each of the print circuit boards 11a-11n carry thereon a wiring pattern 11w, and the interconnection leads 13 are held on a common lead frame 14 as a part thereof. Further, the lead frame 14 includes heat sinks 12a-12n respectively in correspondence to the print circuit boards 11a-11n. Although not illustrated explicitly, the heat sinks 12a-12n are connected to the lead frame 14 as a part thereof by a bridging part. As usual, the wiring patterns 11w are coated with a solder paste, which may be applied by screen-printing process, or the like.
In the process of FIGS. 1A and 1B, it should be noted that the print circuit boards 11a-11n are mounted upon the respective heat sinks 12a-12n in alignment with the lead frame 14 such that interconnection pads formed on the print circuit boards 11a-11n establish a contact engagement with corresponding interconnection leads 13. After the alignment is archived as such, the lead frame 14 and the print circuit boards 11a-11n thereon are passed through a reflowing furnace to achieve a soldering of the interconnection leads 13 upon the corresponding interconnection pads of the print circuit boards 11a-11n.
After the print circuit boards 11a-11n are thus connected firmly upon the lead frame 14 by way of the interconnection leads 13, electric and/or electronic components such as resistors, capacitors, transistors, integrated circuits, and the like, are mounted upon respective parts of the conductor patterns 11w on the print circuit boards 11a-11n, by a robot or other suitable automatic assembling apparatus. As the same assembling procedure is repeated in each of the print circuit boards 11a-11n, the assembling process of FIGS. 1A and 4B is particularly suitable for automatic assembling conducted at high throughput.
The process of FIGS. 1A and 1B, however, has a drawback in that the alignment between the print circuit boards 11a-11n and the interconnection leads 13 on the lead frame 14 may be lost at the time of reflowing of the solder alloy as indicated in FIG. 2.
Referring to FIG. 2, it will be noted that the print circuit board 11a is rotated with respect to the lead frame 14 and hence the interconnection leads 13, while the print circuit boards 11b and 11n are translated parallel with respect to the lead frame 14. In such a case, while the electrical connection between the interconnection leads 13 and the corresponding interconnection pads on the print circuit boards 11a-11n may be maintained, the mounting of the electric and/or electronic components upon the print circuit boards in exact alignment with the wiring patterns 11w thereon becomes difficult. It should be noted that the print circuit boards 11a-11n carry very fine conductor patterns for the wiring patterns 11w for increased mounting density of the components thereon. When the amount of displacement of drifting of the print circuit boards 11a-11n is excessive, even the electrical connection between the interconnection leads 13 and the corresponding pads would no longer be maintained.
Conventionally, therefore, it has been necessary to measure the deviation of each of the print circuit boards 11a-11n accurately by a sensor provided on the robot and to correct the deviation such that the electronic components are properly mounted upon the displaced print circuit boards. However, such a process of detecting and correcting the deviation requires time even when a fully automated assembling apparatus is used, and the throughput of production of the electronic apparatus is inevitably deteriorated.
Alternatively, one may first mount the components on each of the print circuit boards 11a-11n with high precision, followed by mounting each of the print circuit boards 11a-11n on the lead frame 14. Thereby, it is expected that the error between the components and the wiring patterns on the print circuit board is minimized, even if the relative positioning between the interconnection leads 13, and hence the lead frame 14, and the print circuit boards 11a-11n is deteriorated as in the case of FIG. 2.
On the other hand, such a conventional process has a drawback in that the solder alloy on the print circuit board causes a reflowing at the time of mounting the interconnection leads 13 on the print circuit boards 11a-11n, and the components already soldered upon the print circuit boards in exact alignment may experience an unwanted drifting as a result. When such a drifting occurs, the alignment between the components and corresponding wiring patterns 11w on the print circuit may be lost.
In order to avoid such a drifting of the components on the print circuit board, it is necessary to use a different solder alloy composition having a lower melting temperature for the second time soldering process and to carry out the second time soldering at a lower temperature, while use of such additional solder alloy composition or lower temperature furnace for reflowing inevitably increases the cost of the product.